1. Field of the Invention
The invention relates to processes for purifying proteinaceous solutions for enzymatic hydrolysis, particularly to provide organoleptically desirable protein hydrolysate compositions which can be used for dietary purposes and in particular for hospital diets.
2. The Prior Art
Synthetic or low-residue diet foods have been prepared by the prior art to provide the essential nutritional requirements of humans in an easily digestible form. Such diets are predigested protein which have high nutritional value, preferably low ash, and are primarily designed for pre- or postoperative patients or for patients with digestive problems, such as the incapability of breaking down whole protein in the digestive tract (a sympton of cystic fibrosis). One of the primary problems with respect to such diets is palatability of the product.
It is well known that when proteins are digested with strong acid or alkali or with enzymes, hydrolysis of the protein takes place with the subsequent formation of protein fragments, peptides and amino acids. Fragmented protein materials of this type are desirable products for administration to humans (and animals) with digestive problems.
Of the known means to hydrolyze protein, enzymatic hydrolysis is preferred since it does not destroy essential amino acids that are destroyed by acid or alkaline hydrolysis. However, enzymatic hydrolysis rarely goes to completion and the products of the enzymatic hydrolysis cannot be predicted and frequently the hydrolyzed protein is unsuitable because it contains bitter-tasting peptides. Products of this type to be effective as diets must be organoleptically acceptable. (Dietary Enzymatic Hydrolysates of Protein with Reduced Bitterness, Clegg et al., J. Food Tech. (1974) 9, 21-29).
In U.S. Pat. No. 3,857,966 there is disclosed a method for preparing an egg albumen hydrolysate which does not contain the characteristic egg smell and taste. Egg albumen in a 5% by weight solution whose pH is adjusted to 6.3-6.4 is heated to about 85.degree. C. for about 5 minutes to precipitate the protein. After cooling to room temperature, the precipitate is separated by centrifugation and the supernatant is discarded. The precipitate is resuspended in fresh water at pH 6.3-6.5, homogenized in a Waring blender and centrifuged again, discarding the supernatant. The washing step is repeated once more and the wash precipitate is used to make a 5% protein suspension for hydrolysis.
The protein suspension as prepared above is then heated at an alkaline pH (pH 8-9) at a temperature (95.degree.-10020 C.) and for a period of time (generally about 1 hour) effective to condition the protein for efficient enzymatic hydrolysis. Apparently at this stage residual enzyme inhibitors which remain from prior treatments are destroyed. The enzymatic hydrolysis is then conducted using a two-stage enzyme system of an alkaline microbial protease in the first stage and a blend of neutral microbial protease and a plant enzyme in the second stage. Similar techniques are indicated in the patent to be applicable to soy protein isolate, whey or whey protein and fish protein. This process requires a preparation step, a heating or conditioning step and the use of three different enzymes to effect the preparation of an organoleptically acceptable hydrolysate.
U.S. Pat. No. 4,107,334 uses a similar precipitation, technique to prepare the functional protein from microbial, or vegetable protein, or whey by hydrolysis. Generally a solution of low solids and protein content is preferably adjusted to a pH of about the isoelectric point of the protein (4-7) and heated until a large proportion of the protein (at least 50%) has been precipitated (for whey protein concentrate -90.degree. C. for 2 minutes).
After washing, the protein is hydrolyzed using any acid, neutral or alkaline protease (fungal protease suggested).
The value of a protein hydrolysate in a special diet program depends on the degree of hydrolysis, flavor, and the ash content. The prior art processes which relate to precipitation depend upon the reaction of the protein at the isoelectric point. The agents for adjusting pH tend to increase the ash content of the final product. It would be more desirable to have a process which could operate at the native pH of the protein without the need to add pH adjustment agents and their attendant ash content.
In numerous processes, the extent of the hydrolysis reaction is extremely short to avoid extensive hydrolysis which will produce strongly flavored agents. Long hydrolysis time can pose microbiological control problems which require the addition of a preservative Some of these problems are avoided by conducting a partial hydrolysis. The use of a partial hydrolysis reaction is also favored because a more complete hydrolysis would require more time than is convenient or economical. It would be advantageous to be able to conduct the hydrolysis rapidly and more completely while avoiding flavor problems and still obtain a product which is low in ash and highly hydrolyzed.
These features can be provided in accordance with the present invention.